Off-Frequency BMLD: The Role of Monaural Processing

Abstract

Large binaural masking-level differences (BMLDs) can be observed when a tonal signal with an interaural phase difference of π is presented against a diotic masker. The BMLD is large when the signal is spectrally centered in the masker and decreases strongly for off-frequency signals. No such reduction in BMLD would be expected, if monaural detection were governed by energy cues and binaural detection by changes in interaural cross-correlation. The reduction in BMLD thus suggests either that binaural processing is impaired or, alternatively, that additional monaural cues are available in off-frequency conditions. In this study, a stimulus paradigm is used that is expected to impair the processing of additional monaural cues. In the base experiment, a 25-Hz-wide band of diotic noise centered at 700 Hz served as masker. A target tone was presented at 0, 30, 60, and 100 Hz above the masker center frequency, either interaurally in phase (S ₀) or out of phase (S _π). In the extended experiment, an additional interference tone was always presented spectrally below the masker at the same frequency distance as the target tone was positioned above the masker. The interferer level was 6 dB below the level of the 65 dB masker. By presenting the interferer, a strong modulation is introduced, which should impair the detectability of the target tone based on the beating of masker and target. Results show a small off-frequency BMLD in the base experiment in line with literature. Adding the interference tone produced an increase in both N ₀ S ₀ and the N ₀ S _π thresholds, suggesting that monaural modulation cues were indeed used, but also ­suggesting that detection performance in the N ₀ S _π condition was dominated by monaural processing. Additional conditions with modulated interference tones at 500 Hz further supported our hypothesis that monaural modulation cues contributed to reduced ­off-frequency BMLDs.